Untested code in typing.py

[AlexWaygood]

Bug report

If you apply this diff to typing.py, all tests continue to pass:

diff --git a/Lib/typing.py b/Lib/typing.py
index 1dd9398344..98e19644a2 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -1931,6 +1931,7 @@ def _proto_hook(other):
                     if (isinstance(annotations, collections.abc.Mapping) and
                             attr in annotations and
                             issubclass(other, Generic) and getattr(other, '_is_protocol', False)):
+                        1/0
                         break

The break on line 1934 here is unreachable; thus, this whole block of code is pointless:

cpython/Lib/typing.py

Lines 1929 to 1934 in 3af2dc7

	# ...or in annotations, if it is a sub-protocol.
	annotations = getattr(base, '__annotations__', {})
	if (isinstance(annotations, collections.abc.Mapping) and
	attr in annotations and
	issubclass(other, Generic) and getattr(other, '_is_protocol', False)):
	break

I think we can say for sure that the break here is unreachable. This is the __subclasshook__ method that is monkey-patched onto all subclasses of typing.Protocol that do not define their own __subclasshook__ methods. This block of code in the __subclasshook__ method is inspecting the __annotations__ dictionary of other to see if it can find any protocol members in that dictionary. But we know that there can't be any protocol members in the __annotations__ dictionary, because if there were, that would make other a protocol with at least one non-callable member. If it's a protocol that has at least one non-callable member, the __subclasshook__ method is never called at all during isinstance() or issubclass() checks, because we raise TypeError in _ProtocolMeta.__subclasscheck__, short-circuiting the call to abc.ABCMeta.__subclasscheck__ that would call Protocol.__subclasshook__:

cpython/Lib/typing.py

Lines 1828 to 1831 in 3af2dc7

	if not cls.__callable_proto_members_only__:
	raise TypeError(
	"Protocols with non-method members don't support issubclass()"
	)

I believe that this block of code can therefore be safely deleted.

Linked PRs

• gh-105834: Add tests for calling issubclass() between two protocols #105835
• [3.12] gh-105834: Add tests for calling issubclass() between two protocols (GH-105835) #105859
• [3.11] gh-105834: Backport new tests for typing.Protocol (#105835) #105860

[AlexWaygood]

As has been pointed out in the PR discussion, this code is not in fact dead, just completely untested. (Thanks @JelleZijlstra!)